use time.IsZero in Prune
use time.IsZero in Prune

Signed-off-by: u5surf <9f68a971eeb1225a45cda60aafda53ca3aaad15f@gmail.com>

go-git/go-git,mcuadros/go-git,mcuadros/go-git,src-d/go-git,src-d/go-git,go-git/go-git